The invention relates to a method of welding single crystals on a monocrystalline substrate and to the use of the method for supplementing a single crystal surface structure of a worn single crystal substrate.
It is known to manufacture metallic workpieces subject to high mechanical, chemical or thermal stresses not by classical casting processes, but to grow them in the form of single crystals from the metallic melt. In this connection, well-known methods (which can be suitably modified) such as the Bridgman process are used. This means that the workpiece is obtained in the desired form in a controlled manner by directional solidification from a metal melt.
When single crystals or single crystal areas, layers or structures are spoken of in the present specification, these include single crystals which have no grain boundaries or, at the most, small-angle grain boundaries of less than 10°. The previously named terms can thus also be understood to include so-called “directionally solidified structures”, with these being single crystal structures which have grain boundaries which are, however, substantially parallel, i.e. only extend in one direction.
Such metallic workpieces having a single crystal structure are preferably used as machine components which are exposed to high temperatures and severe mechanical or chemical stresses over longer periods of time. For instance, as a rule the melting temperature of the single crystal structures can, for example, be substantially increased with respect to the melting temperature of a corresponding polycrystalline workpiece manufactured of the same material. The melting point is moreover defined much more sharply in the single crystal workpieces.
For this reason, single crystal machine parts are e.g. particularly suitable for use as blades of gas turbines or as rotors for aircraft power plants. So-called “super-alloys”, e.g. on a nickel (Ni) or on a cobalt (Co) or on an iron (Fe) base, have proved themselves as alloys for such single crystal turbine blades. Particularly super-alloys on a nickel base such as CMSX-2 or CMSX-4 show exceptional mechanical, chemical and thermal properties even at high temperatures only a little below their melting point.
In spite of the previously described high durability of such moncrystalline machine parts, a certain wear of the surface occurs in operation over time due to the high stresses. That is, in operation, grooves, stepped shoulders, depressions or damage of a different kind, for example, can occur at the surface of such machine parts. It can even occur that, e.g. due to friction or other wear processes, e.g. of a chemical or thermal kind, whole surface regions are removed over a large area so that the corresponding machine parts either have to be replaced or their damaged surfaces have to be restored. Since the manufacture of complete workpieces in complex single crystal growth processes is very cost-intensive, it is usually preferable to restore damaged surfaces by suitable repair methods.
In this connection, the restoration (reconditioning), that is, the repair of the surfaces, has to take place by the rebuilding of single crystal layers or by supplementing single crystal areas. That is, the original structure is built up again on the substrate, e.g. on a single crystal turbine blade, layer for layer in a single crystal manner in the damaged areas (preferably using the same material of which the substrate is made). In this connection, the crystalline structure of the substrate must be continued without grain boundaries where possible. Such methods are described by the term “single crystal welding” within the context of this application.
A method of single crystal welding is known from EP 0 861 927 A1 with which it is possible to build up one or more layers, or a body or a workpiece with a single crystal structure, on the single crystal structure of a substrate. This method is an epitaxial method; i.e. the crystalline structure of the substrate is adopted by the layer which is being built up on it. Generally, epitaxy, or epitaxial growth, is understood as crystal growth in one direction, in particular on a single crystal base, i.e. on a substrate. Laser beams can for example also be used as energy sources or heat sources for the carrying out of these methods. It is possible with these to introduce high amounts of energy in a concentrated manner on a very small area. In the method shown in EP 0 861 927 A1, a beam of high energy and of high energy density is directed in a concentrated manner onto the surface of the substrate so that the surface layer of the substrate starts to melt a little. The required material for the reconstruction of the surface is supplied to the working area of the concentrated beam in the form of a wire or in the form of powder and is likewise melted by the beam of energy. With the known method of single crystal welding, a situation can be achieved by a suitable selection of the process parameters in which the melted material solidifies directionally, starting from a surface of the substrate, and grows on the surface of the substrate as an epitaxial layer.
This known method, which has successfully proven itself in principle in practice to date, is, however, still capable of improvement in one point. In the known method, the process of single crystal growth namely takes place such that the melted material directionally solidifies from all adjoining areas (i.e. adjoining the melted region). This means that different epitaxial growth areas can arise which can be oriented at a certain angle with respect to one another in accordance with the adjoining areas of the surface of the substrate. Within the framework of this application, such a growth is termed a “multi-axial growth”. The term “uni-axial growth” is used in an analogous manner. These differently oriented growth regions can then meet at one or more interfaces, with grain boundaries then being able to form at said interfaces. Such grain boundaries between growth areas having different orientations with respect to one another therefore particularly occur when the surface is restored at stepped shoulders, at grooves or at depressions of the single crystal substrate. Problems of the kind described above can also occur on the application of large area layers to the single crystal substrate. In practice, the energy beam is guided over the substrate at a pre-settable speed so that narrow regions are produced in the form of tracks approximately of the width of the laser beam. In order to apply large area layers, a plurality of parallel tracks, directly adjoining one another, must thus be produced on the surface of the substrate by means of the laser beam, such that one is confronted with a step-like structure which favors epitaxial growth in different directions at the same time on the application of each further track (along a previously laid track).